As a related art in which a multi-link-type piston crank mechanism connects between a piston pin and a crank pin of a reciprocating internal combustion engine, a patent document 1 previously suggested by the present applicant has been commonly known. This includes an upper link connected to a piston pin of a piston, a lower link connecting the upper link with a crank pin of a crankshaft and a control link whose one end is swingably supported at an engine body side and the other end is connected to the lower link. The upper link and the lower link are rotatably connected to each other via an upper pin, and the control link and the lower link are rotatably connected to each other via a control pin.
Here, to realize the attachment of the lower link to the crank pin, the lower link is configured by dividing it into two parts of a lower link upper with which the upper link is connected and a lower link lower with which the control link is connected. These lower link upper and lower link lower are combined so as to sandwich the crank pin, and as shown in a patent document 2, they are fastened to each other by a pair of bolts inserted from directions opposite to each other.
In a case of a general single-link-type piston crank mechanism as a piston crank mechanism in a reciprocating internal combustion engine, the large end part of a connecting rod whose small end part is connected to a piston is divided into a main body part and a cap part, and a crank pin is sandwiched, and they are fastened to each other by a pair of bolts arranged parallel to each other.
This fastening for the bolts, that is, the assembling of the connecting rod and a crankshaft is generally carried out in a state in which a cylinder block is inverted and a crank case is directed upward. That is, the main body part of the connecting rod is connected with the piston in advance, and after each piston is inserted into a corresponding one of cylinders, the crankshaft is placed on main bearing parts of the cylinder block set in an inverted position, and cap portions are placed on respective crank pins of the crankshaft, and then the fastening with the main body parts protruding upward from the respective cylinders is carried out. A pair of the bolts is inserted and disposed parallel to each other, and then fastened by a nut runner.
For example, in an in-line four-cylinder internal combustion engine, the crank pins of #1 and #4 cylinder sections are disposed at positions different in phases by 180° from the crank pins of #2 and #3 cylinder sections. However, the fastening of the bolts can be carried out from above with respect to all of the cylinder sections, and basically, the fastening of the bolts of all of the cylinder sections is therefore possible without rotating the crankshaft.
As compared with a case of the single-link-type piston crank mechanism, in the above-mentioned multi-link-type piston crank mechanism, the fastening work of a lower link to a crankshaft (crank pin) becomes quite complicated. The lower link is fastened by bolts arranged parallel to and opposite to each other. Moreover, since there are a plurality of crank pins having different phases, to fit the lower links to the crank pins of all of the cylinder sections in the state in which the cylinder block is set in an inverted position, it is necessary that the positions of the lower links are rotated vertically while the crankshaft is rotated plural times by a predetermined angle each time (for example, 180° each, in an in-line four-cylinder engine), and the fastening of each of the bolts is carried out. Furthermore, a plurality of steps to one bolt is needed for bolt axial force control. For example, steps are carried out from the application of snug torque through returning to torque zero and the application of the snug torque to the angle fastening of a predetermined angle. If each of these steps is alternately carried out to two bolts of each of the lower links, bolts which face opposite to each other, the number of times of the rotation operation of the crankshaft further increases, and, for example, a few tens of the rotation operations are needed.
Therefore, an object of the present invention is to provide a bolt fastening method for lower links with which fastening work of a pair of bolts arranged opposite to each other and fitted to each of the lower links can be easily carried out by using nut runners.